Much research has been carried out to correlate the surface charges to particle capture in papermaking and filtration industries. Most of the previous works model the co-filtration process of simultaneous fibrous mat formation and filler particle capture as a sequential combination of cake and depth filtration. Very few works have quantified the surface charge and related it to the filtration behavior. The objective of this work is to model the co-filtration process through multiphase transport theory, coupled with the postulated constitutive equations and experimental data, to account for the effect of surface charges on filtration efficiency and to explain the dominant particle capture mechanism under different water chemistry conditions. The model proposed here is compared with experimental data from a co-filtration process of raw paper formation using cellulose fibers, TiO2 particles as the fillers, and alum as the salt additive. The zeta potentials, i.e., effective surface charges, of cellulose fibers and TiO2, particles under different conditions were measured prior to filtration experiments. The pH, salt and particle concentrations were varied in bench and pilot filtration experiments. The results of filtration experiments show a significant effect of material surface charges on particle capture efficiency. The filtration efficiency increased between 30% to over 50% by adding the salt to the slurries. The model equations with constitutive functions were numerically solved and the capture coefficients were optimized using a genetic algorithm to 'best' fit the experimental data. A FORTRAN program was written to handle the moving boundary condition (function of time) of co-filtration process. A dimensionless zeta potential function was defined to relate the zeta potentials of both the fibers and the filler particles and incorporated into the particle capture relation. The capture relation separates the particle capture due to electrostatic-attraction from other capture mechanisms. The model results show electrostatic-attraction between fibers and fillers to be the dominant particle capture mechanism when salt is added to the slurry.